Sybil

This project provides a set of tools to perform Kerberos protocol transition and ticket impersonation.

It is comprised of:

• A privileged daemon hosted alongside the KDC which provides impersonation services (sybild)
• A command line interface which can be used to request tickets on behalf of users (sybil)

When used together, this enables services which provide their own authentication mechanism to utilize a Kerberized infrastructure.
For example, this can allow a CI/CD agent to submit pipelines on behalf of a user authenticated outside of the Kerberos realm (e.g. OIDC, SAML).

This project relies among other things on GSSAPI and the Microsoft S4U protocol extensions.

Build

Binaries

# Prerequisites
curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh
apt install clang libkrb5-dev

make release

Packages

# Prerequisites
cargo install cargo-generate-rpm cargo-deb

make deb rpm

Setup

IPA

Assuming an existing FreeIPA or RHEL IdM install:

# Create the Sybil service principal and generate its keytab
ipa service-add sybil/ipa.domain.lan
ipa-getkeytab -p sybil/ipa.domain.lan -k /etc/krb5.keytab

# Create the Sybil DNS service record
ipa dnsrecord-add --srv-priority=0 --srv-weight=100 --srv-port=57811 --srv-target=ipa.domain.lan. domain.lan _sybil._tcp

# Allow a host to perform impersonation against the Sybil service
ipa servicedelegationtarget-add sybil-target
ipa servicedelegationtarget-add-member --principals sybil/ipa.domain.lan sybil-target
ipa servicedelegationrule-add sybil
ipa servicedelegationrule-add-member --principals host/server.domain.lan sybil
ipa servicedelegationrule-add-target --servicedelegationtargets=sybil-target sybil
ipa host-mod --ok-to-auth-as-delegate=true server.domain.lan

# Configure and run Sybil
cat > /etc/sybil.toml <<EOF
tkt_cipher = "aes256-sha1"
tkt_flags = "FRI"
tkt_life = "10h"
tkt_renew_life = "7d"
allow_networks = ["192.168.0.0/24"]
allow_realms = ["DOMAIN.LAN"]
allow_groups = ["[email protected]"]
strip_domain = true
cross_realm = ""
EOF

systemctl enable --now sybil

MIT Kerberos

Assuming an existing MIT Kerberos install with the LDAP backend (required for S4U to work)

# Create the Sybil service principal and generate its keytab
kadmin.local addprinc -randkey sybil/mit.domain.lan
kadmin.local ktadd -k /etc/krb5.keytab sybil/mit.domain.lan

# Create the Sybil DNS service record as follow in your DNS server
# _sybil._tcp.domain.lan. 86400 IN SRV 0 100 57811 mit.domain.lan.

# Allow a host to perform impersonation against the Sybil service
ldapmodify -Y EXTERNAL -H ldapi:// <<EOF
dn: krbPrincipalName=host/[email protected],cn=DOMAIN.LAN,cn=krbContainer,dc=domain,dc=lan
changetype: modify
add: krbAllowedToDelegateTo
krbAllowedToDelegateTo: sybil/mit.domain.lan
EOF
kadmin.local modprinc +ok_to_auth_as_delegate host/server.domain.lan

# Configure and run Sybil
cat > /etc/sybil.toml <<EOF
tkt_cipher = "aes256-sha1"
tkt_flags = "FRI"
tkt_life = "10h"
tkt_renew_life = "7d"
allow_networks = ["192.168.0.0/24"]
allow_realms = ["DOMAIN.LAN"]
allow_groups = ["[email protected]"]
strip_domain = true
cross_realm = ""
EOF

systemctl enable --now sybil

Usage

# Retrieve a ticket for the host
kinit -k

# Acquire a ticket on behalf of the given principal
sybil kinit [email protected]

# Verify that a ticket has been acquired
sudo -u user klist

Default principal: [email protected]

Valid starting       Expires              Service principal
09/15/2023 00:38:23  09/15/2023 10:38:23  krbtgt/[email protected]
        renew until 09/22/2023 00:38:23